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Ion adsorption and transfer at charged interfaces play key roles in various industrial and environmental processes. Molecular scale details of ion–ion, ion–water, and ion–surface interactions are still debated. Complex ions, such as SCN–and SeCN–, are particularly interesting due to their unexpected adsorption trends. Here, we combine vibrational sum frequency generation (VSFG) spectroscopy and surface-sensitive synchrotron X-ray studies to provide a detailed description of SeCN–adsorption at a floating charged monolayer. Polarimetry studies show that the average orientation of SeCN–anions with respect to the surface normal decreases from 45° to 22° with the increasing KSeCN concentration. Interfacial SeCN–coverage saturates at very low bulk concentrations, but their orientational organization, distribution between Stern and diffuse layers, and effects on the hydrogen-bonding network of the interfacial water continue to change with increasing bulk KSeCN concentration. These results show that the increasing chemical potential may lead to further reorganization of the adsorbed ions, even though the total interfacial ion population does not change. The reorganization of the interfacial ions and the water may be very important in chemical separations of heavy metals, where metal–anion complexes drive the selective ion transfer at aqueous interfaces.

Effective and energy-efficient separation of precious and rare metals is very important for a variety of advanced technologies. Liquid–liquid extraction (LLE) is a relatively less energy intensive separation technique, widely used in separation of lanthanides, actinides, and platinum group metals (PGMs). In LLE, the distribution of an ion between an aqueous phase and an organic phase is determined by enthalpic (coordination interactions) and entropic (fluid reorganization) contributions. The molecular scale details of these contributions are not well understood. Preferential extraction of an ion from the aqueous phase is usually correlated with the resulting fluid organization in the organic phase, as the longer-range organization increases with metal loading. However, it is difficult to determine the extent to which organic phase fluid organization causes, or is caused by, metal loading. In this study, we demonstrate that two systems with the same metal loading may impart very different organic phase organizations and investigate the underlying molecular scale mechanism. Small-angle X-ray scattering shows that the structure of a quaternary ammonium extractant solution in toluene is affected differently by the extraction of two metalates (octahedral PtCl62–and square-planar PdCl42–), although both are completely transferred into the organic phase. The aggregates formed by the metalate–extractant complexes (approximated as reverse micelles) exhibit a more long-range order (clustering) with PtCl62–compared to that with PdCl42–. Vibrational sum frequency generation spectroscopy and complementary atomistic molecular dynamics simulations on model Langmuir monolayers indicate that the two metalates affect the interfacial hydration structures differently. Furthermore, the interfacial hydration is correlated with water extraction into the organic phase. These results support a strong relationship between the organic phase organizational structure and the different local hydration present within the aggregates of metalate–extractant complexes, which is independent of metalate concentration.

Presented here is an overview of nonvolatile particulate matter (nvPM) emissions, i.e., “soot”, as assessed by TEM analyses of samples collected after the exhaust of a J-85 turbojet fueled with Jet-A as well as with blends of Jet-A and Camelina biofuel. A unifying explanation is provided to illustrate the combustion dynamics of biofuel and Jet-A fuel. The variations of primary particle size, aggregate size, and nanostructure are analyzed as a function of biofuel blend across a range of engine thrust levels. The postulate is based on where fuels start along the soot formation pathway. Increasing biofuel content lowers aromatic concentration while placing increasing dependence upon fuel pyrolysis reactions to form the requisite concentration of aromatics for particle inception and growth. The required “kinetic” time for pyrolysis reactions to produce benzene and multiring PAHs allows increased fuel–air mixing by turbulence, diluting the fuel-rich soot-forming regions, effectively lowering their equivalence ratio. With a lower precursor concentration, particle inception is slowed, the resulting concentration of primary particles is lowered, and smaller aggregates were measured. The lower equivalence ratio also results in smaller primary particles because of the lower concentration of growth species.

A fundamental understanding of the factors that determine the interactions with and transport of small molecules through phospholipid membranes is crucial in developing liposome-based drug delivery systems. Here we combine time-dependent second harmonic generation (SHG) measurements with molecular dynamics simulations to elucidate the events associated with adsorption and transport of the small molecular cation, malachite green isothiocyanate (MGITC), in colloidal liposomes of different compositions. The molecular transport of MGITC through the liposome bilayer is found to be more rapid in 1,2-dioleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) and 1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPG and DOPS, respectively) liposomes, while the molecular transport is slower in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes. Interestingly, MGITC is observed to neither adsorb nor transport in trimethyl quinone-1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (QPADOPE) liposomes due to shielding by the quinone group. The modified Langmuir adsorption isotherm model is used to determine the free energy of adsorption for MGITC, which is found to be less negative in DOPC than in DOPG and DOPS, caused by lower electrostatic interactions between the positively charged dye and the zwitterionic DOPC liposome surface. The results are compared to our previous investigations, which showed that malachite green (MG) adsorbs and transports in DOPG and DOPS liposomes but not in DOPC and QPADOPE liposomes. Molecular dynamics simulations are used to investigate the adsorption and transport properties of MG and MGITC in DOPC and DOPG liposomes using umbrella sampling to determine the free energy profiles and interfacial molecular orientations. Together, these time-resolved SHG studies and corresponding molecular dynamics simulations characterize the complicated chemical interactions at different lipid membranes to provide key molecular-level insights for potential drug delivery applications. The results also point toward understanding the role of chemical functional groups, in this case isothiocyanate, in controlling molecular adsorption at and transport through lipid bilayers.

In situsecond harmonic generation (SHG) coupled with extinction spectroscopy is used for real-time monitoring of seed-mediated growth dynamics of colloidal citrate-stabilized gold nanoparticles in water. The time-dependent in situSHG results capture an early stage of the growth process where a large enhancement in the SHG signal is observed, which is attributed to the formation of plasmonic hot spots from a rough and uneven nanoparticle surface. The temporal peak in the SHG signal is followed by a decay that is fit to an exponential function to characterize the size-dependent nanoparticle growth lifetime, which varies from 0.45 to 1.7 min for final nanoparticle sizes of 66 and 94 nm, respectively. This early growth stage also corresponds to a broadening of the plasmon spectra, as monitored using time-dependent in situextinction spectroscopy. Over the course of the seed-mediated growth reaction, the nanoparticle becomes more thermodynamically stable through surface reconstruction resulting in a smoother, more uniform surface, corresponding to lower, stable SHG signals and narrower plasmon spectra. With real-time monitoring of nanoparticle formation, in situSHG spectroscopy combined with in situextinction spectroscopy provides an important insight for controlling nanoparticle synthesis and surface morphology for potential nanoscale engineering of different colloidal nanomaterials.

Photothermal release of oligonucleotides from the surface of plasmonic nanoparticles represents a promising platform for spatiotemporal controlled drug delivery. Here we demonstrate the use of novel gold–silver–gold core–shell–shell (CSS) nanoparticles to study the photothermal cleaving and release of micro-RNA (miRNA) mimics or small interfering RNA (siRNA) under near-infrared (NIR) irradiation. The furan–maleimide-based Diels–Alder adduct cleaves thermally above 60 °C and is used to bind siRNA to the colloidal nanoparticle surface in water. We investigate the photothermal cleaving kinetics of siRNA under different NIR laser powers using surface-sensitive time-dependent second-harmonic generation (SHG) spectroscopy. The photothermal release of siRNA from the surface of CSS nanoparticles is significantly higher than that from the surface of gold nanoparticles (GNPs) under similar experimental conditions. These results demonstrate that plasmonic CSS nanoparticles with photothermal cleaving linkers have important potential applications for nanoparticle-based NIR-mediated drug-delivery systems.

It is of great importance to interrogate the impact of local environment on the transport of small molecules across lipid bilayers, as they are key to the function and capabilities of eukaryotic cells and liposome-based delivery systems. Herein are described real-time studies of the molecular adsorption and transport kinetics of positively charged small-molecule organic dyes at the surface of liposomes under different buffer and salt conditions, made possible by application of second harmonic generation (SHG). The molecular transport of malachite green (MG) within the liposome bilayer is more rapid in 1,2-dioleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) and 1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPG and DOPS, respectively) liposomes in citrate buffer without added salts, whereas no adsorption or transport of MG is observed in trimethyl quinone-1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (QPADOPE) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes. While the transport rate constant increases linearly with concentration of MG for DOPG liposomes, there is much less dependence of the transport rate constant on the concentration of MG in DOPS liposomes. Adsorption site densities and free energies of adsorption are determined using the modified Langmuir adsorption isotherm model by fitting SHG signals resulting from MG addition. The resulting free energy of adsorption for MG to the surface of liposomes is higher in the absence of added salts due to increased electrostatic attractions. However, the corresponding adsorption site densities increase in the presence of salts due to ion-pair formation and decreased repulsions between cationic adsorbates. Comparisons at different citrate buffer and KCl concentrations demonstrate that adsorption and transport of MG in liposomes are influenced by several interrelated factors that include molecular structure of the lipid headgroup, electrostatic interactions between the charged liposome surface and ionic adsorbates and electrolytes, and ion-pair formation. Similar investigations using the structurally similar but more highly charged dicationic dye methyl green point to a lack of its adsorption to and transport within the liposomes, possibly due to a much stronger hydration shell. These findings highlight important considerations for potential liposome-based, drug-delivery applications and the transport of small-molecule drugs across the plasma membrane.

Second harmonic generation is used to investigate the surface charge density of 50 nm colloidal gold nanoparticles in water. The gold nanoparticles are thiolated with mercaptosuccinic acid and are dialyzed in ultrapure water to remove excess salts and reactants. The second harmonic generation signal from the nanoparticle sample is measured as a function of added sodium chloride and magnesium chloride salt concentrations using the χ(3)technique. The experimental results are fit to the Gouy–Chapman model and to numerical solutions to the spherical Poisson–Boltzmann equation that account for the nanoparticle surface curvature, the different salt valences, and ion adsorption to the Stern layer interface. The best fits use the numerical solutions including ion adsorption and determine the initial surface charge density to be (−2.0 ± 0.1) × 10–3C/m2at the gold nanoparticle surface, in agreement with electrophoretic mobility measurements. In addition, the sodium ion is observed to adsorb with a higher surface charge density than the magnesium ion. These results demonstrate the important effects of surface curvature and ion adsorption in describing the surface chemistry and surface charge density of colloidal gold nanoparticles in water.

Graphene oxide (GO) membranes are excellent candidates for a range of separation applications, including rare earth segregation and radionuclide decontamination. Understanding nanoscale water and ion behavior near interfacial GO is critical for groundbreaking membrane advances, including improved selectivity and permeability. We experimentally examine the impact of solution conditions on water and lanthanide interactions with interfacial GO films and connect these results to GO membrane performance. The investigation of the confined films at the air–water interface with a combination of surface-specific spectroscopy and X-ray scattering techniques allows us to understand water and ion behaviors separately. Sum frequency generation spectroscopy reveals a dramatic change in interfacial water organization because of graphene oxide film deprotonation. Interfacial X-ray fluorescence measurements show a 17× increase in adsorbed lanthanide to the GO film from subphase pH 3 to pH 9. Liquid surface X-ray reflectivity data show an additional 2.7 e–per Å2for GO films at pH 9 versus pH 3 as well. These results are connected to GO membrane performance, which show increased selectivity and decreased flux for membranes filtering pH 9 solutions. We posit insoluble lanthanide hydroxides form at higher pHs. Taken together, these results highlight the importance of interfacial experiments on model GO systems.

Developing better separation technologies for rare earth metals, an important aspect of a sustainable materials economy, is challenging due to their chemical similarities. Identifying molecular-scale interactions that amplify the subtle differences between the rare earths can be useful in developing new separation technologies. Here, we describe the ion-dependent monolayer to inverted bilayer transformation of extractant molecules at the air/aqueous interface. The inverted bilayers form with Lu3+ions but not with Nd3+. By introducing Lu3+ions to preformed monolayers, we extract kinetic parameters corresponding to the monolayer to inverted bilayer conversion. Temperature-dependent studies show Arrhenius behavior with an energy barrier of 40 kcal/mol. The kinetics of monolayer to inverted bilayer conversion is also affected by the character of the background anion, although anions are expected to be repelled from the interface. Our results show the outsized importance of ion-specific effects on interfacial structure and kinetics, pointing to their role in chemical separation methods.